1. Field of the Invention
This invention relates to the free casting of metals and to the filaments which are thereby produced.
2. Description of Prior Art
The preparation of metal wires by drawing through a die is an old art which is known to have been practiced by hand at least as early as 1000 A.D. Metal filaments are still principally produced by pulling a large diameter rod through a succession of tapered dies, each one progressively smaller than the one before. In modern practice, the die drawing process has been mechanized, made continuous and automated; yet, the process remains essentially unchanged as do a number of associated problems and limitations.
Research in recent years has been directed toward the development of methods of filament formation which avoid the restrictions of die drawing. One of the approaches under investigation involves free casting or direct melt spinning and concerns the formation of a free jet of molten fluid and the transformation of the jet to the solid state. This procedure may be employed to form filaments of polymeric materials and glasses, i.e. materials having very high viscosities and low surface tension in the liquid state. In contrast, however, metals have relatively inviscid melts of high surface free energy. A cylindrical jet of such a material is inherently unstable. Its surface becomes increasingly perturbed as it issues from the nozzle until at some distance the jet breaks up into droplets. Accordingly, a process, if it is to be capable of producing continuous filaments of metals, must provide a favorable balance between the kinetics of jet solidification and of jet breakup.
Although several processes for the melt spinning of metal fibers have been proposed to effect a stabilization of the molten jet as a means of achieving the required balance, no process has been significantly successful. In one process, the jet is extruded into a gaseous atmosphere capable of chemical reaction with one or more of the components of the jet. Stabilization occurs by formation of a solid sheath or skin on the liquid jet. Alternatively, electrostatic charges have been used to stabilize the jet.
Several techniques have also been proposed for hastening the solidification of the jet. For example, Pond in U.S. Pat. No. 3,602,291 describes cooling of the molten jet by a mist of a vaporizing liquid dispersed in a gas. Schile (U.S. Pat. No. 3,543,831) employs cooling by a gas-solid dispersion. Engelke et al (U.S. Pat. No. 3,347,959) discloses immersing the nozzle from which the molten jet issued in a "liquid mold stream" maintained at a slightly lower temperature than the melt. However, the solidification rate in each of these processes remains fairly slow and chemical or electrostatic stabilization of the jet is still required. Additionally, when a nozzle is immersed in the cooling medium, serious practical difficulties are present, e.g. unsuitable quenching and corrosion problems.
The need for chemical stabilization of a molten jet imposes several hardships upon a spinning process; among these are the following:
1. The addition of a reactive element to a pure metal or metal alloy may have a detrimental effect on mechanical, electrical or other of its physical properties.
2. Precise control of melt and/or atmospheric compositions is required, lest the attendant chemical reaction causes plugging of the spinning orifice on the one hand or inadequate stabilization of the jet on the other.
3. The choice of crucible and orifice materials which may satisfactorily resist erosion and chemical attack is limited.
4. The reactive gaseous atmosphere may be noxious, inflammable, explosive, corrosive or expensive.
It is well known that there are a number of disadvantages in working with high electrical potentials to effect electrostatic jet stabilization.